The basic operation and structure of communication systems such as cellular radio telephone communication systems and land mobile communication systems are well known in the art. Communication systems typically comprise a plurality of communication units, a predetermined number of base stations (or repeaters) located throughout a geographic region and a controller. The communication units may be vehicle mounted or portable units. The communication units and the base stations each comprise either a transmitter or a receiver or both to form transceiver. The communication units are coupled to the base stations by a communication channel over which modulated signals, such as radio frequency (RF) signals, are transmitted and/or received. The controller comprises a centralized call processing unit or a network of distributed controllers working together to establish communication paths for the communication units in the communication system.
More particularly, a receiver of the communication unit receives a modulated signal subsequent to transmission thereof by a transmitter of the base station on the communication channel. The receiver includes a down converter, a sampler and a detector. The down converter translates the received modulated signal to baseband frequency to produce a down: converted signal. The sampler samples the down converted signal at multiple points in time to produce a sampled signal. The detector detects the sampled signal at one of the multiple points in time to produce a detected signal. The detector is typically a coherent detector or a maximum likelihood sequence estimator (MLSE) detector. The MLSE detector is also sometimes referred to as an MLSE equalizer.
Transmission impairments resulting in inaccurate detection of the modulated signal transmitted on the communication channel are primarily caused by spurious and thermal noise, adjacent and co-channel interference, intersymbol interference and fiat fading due to multipath interference. These transmission impairments sometimes cause the receiver to determine the wrong point in time at which to detect the sampled signal.
Peak correlation is a well known technique that attempts to overcome the transmission impairments for determining the point in time at which to detect the sampled signal. Correlation is a well known technique for estimating an impulse response of the communication channel by correlating, or matching, the modulated signal with a predetermined signal stored in the receiver. The point in time selected to detect the sampled signal corresponds to a peak of the estimated channel impulse response.
Peak correlation performs well under fiat fading channel conditions because it selects the point in time corresponding to the strongest signal component. However, under intersymbol interference channel conditions, the peak correlation technique sometimes selects a suboptimal point in time because the peak correlation technique ignores the level of the intersymbol interference and the effect that the intersymbol interference will have on the detector.
Therefore, there is a need for an improved apparatus and method for determining a point in time for detecting a sampled signal in a receiver for intersymbol interference conditions.